In a hierarchical mobile telecommunications network, the backhaul portion of the network comprises the intermediate links between the core network of the mobile telecommunications network and the small sub-networks at the peripheral of the entire hierarchical network. For example, while user equipment communicating with a base station constitute a local sub-network, the connection between the base station and the rest of the world begins with a backhaul link to the core of the telecommunication supplier's network. A variety of different passive optical networks (PONs) can be used as mobile backhauls. For instance, optical networks such as gigabit-passive optical networks (GPONs), which provide multiple access methods, are suitable mobile backhaul solutions referred to as GPON Radio Access Networks (GPON RAN). However, other PON solutions are possible, such as e.g. Ethernet PON (EPON) or Wavelength Division Multiplexing PON (WDMPON). In the following, GPON will be used to exemplify the xPON networks.
A GPON is a fiber-based access technology illustrated in FIG. 1, where an optical network 10 essentially transports traffic between base stations 11 and a base station controller 12. A plurality of user nodes 15 in the form of optical network units (ONU) provides telecom services to end users via customer-premises equipment (CPE), which generally refers to devices such as telephones, routers, switches, set-top boxes, etc. The ONUs are connected to a central office node 13 in the form of an optical line terminal (OLT), via an optical distribution network 14 (ODN), where the plurality of (up to 64) ONUs share a common splitter and a trunk fiber connected to the OLT. The OLT serves as access multiplexer aggregating traffic from ONUs and distributing traffic towards ONUs. Thus, at the one end of the network, the OLT provides the interface between the GPON and the service provider's network services including e.g. IP traffic such as Ethernet and time division multiplexed (TDM) interfaces such as SONET. At the other end of the network, the ONUs provide the interface between the GPON and the end users, where services include e.g. voice in the form of plain old telephone service (POTS) or voice over IP (VoIP), data such as Ethernet, video, etc.
An ONU contains beside optics a GPON framer for managing packet data as well as a host CPU for controlling the GPON framer as well as Ethernet related logic (switch, traffic management, L3 functions, etc.) and/or voice related logic. The ONU also contains flash memory to store software that can be updated remotely by the OLT.
Recently, highly integrated ONU chipsets have become available containing many more functions in addition to the framer. This allows for making very small ONUs that can be hosted in a small-formfactor pluggable (SFP) module. Advantageously, at the ONU, as soon as bandwidth requirements increase, one or more SFP modules can be plugged in making the ONU highly scalable. Further, different SFPs can be plugged in depending on reach requirements, since different SFP types have different reach (1 km, 5 km, 80 km, etc.)
The SFP is currently the most popular transceiver formfactor, used for Ethernet (1 G), SDH/SONET, Fibre Channel and PONs (EPON, GPON, WDM). The SFP modules are plugged into SFP connectors which are mounted on a main-board of an ONU. The connectors are housed within an EMI cage where the SFP module is connected. SFP has been standardized by the SFF MSA group, and an enhanced version SFP+ is currently used for 10 G Ethernet with the same connector layout.
To make synchronous PONs usable as mobile backhauls, the transport of synchronization signals is a strict requirement. In the art, this has been solved by integrating a packet-based 1588 infrastructure in the SFP module, which is described in detail with reference to FIG. 2. That is, the SFP module has been modified to comply with a precision time protocol described in a standard referred to as IEEE 1588. With this protocol, it is possible to synchronize distributed clocks with an accuracy of less than 1 microsecond via Ethernet networks. However, this approach requires great modification of ordinary off-the-shelf SFP modules, making them both complex and expensive.